The present invention relates to an improvement in a rotary machine, such as electric motors and power generators, with a permanent magnet and, more particularly, to an improvement in a compact-size rotary machine comprising one or more of permanent magnets, a yoke and an armature.
A typical structure of a conventional servomotor, which is a kind of permanent-magent rotary machine, is schematically illustrated in FIGS. 1a and 1b by an axial and a radial cross section, respectively, in which a pair of upper and lower permanent magents 2 and 3 are held symmetrically by the yoke 4,5 each facing the armature 1 rotatable around the shaft 7. The magnetic flux .PHI. forms a loop passing through the "armature 1.fwdarw. upper permanent magnet 2.fwdarw. yoke 4,5.fwdarw. lower permanent magnet 3.fwdarw. armature 1". Assuming that the cross sectional area and magnetic flux density in the magnetic circuit of the permanent magnets 2 and 3 are given by A.sub.m and B.sub.m, respectively, and the cross sectional area and magnetic flux density in the magnetic circuit of one of the wings of the yoke 4,5 are given by A.sub.y /2 and B.sub.y, respectively, then the following equation sets forth the relationship of these parameters: EQU (1/.alpha.).times.A.sub.m .times.B.sub.m =(A.sub.y /2).times.B.sub.y .times.2=A.sub.y .times.B.sub.y, (I)
in which .alpha. is an area factor which usually has a value smaller than 1 in order not to magnetically saturate the yoke 4,5.
When an electric current is passed through each of the armature coils 6, the shaft 7 is rotated with a torque proportional to the magnetic flux. Since the magnetic flux .PHI. is a function of temperature, however, the output of the motor will not always be constant. For example, the output is decreased as a result of a decrease in the magnetic flux .PHI. when the temperature of the motor is increased by the heat evolved from the armature coils 6 or by an increase in the ambient temperature. The instability of the output of the motor is rather great when ferrite magnets, rare earth-based magnets and the like are used as the permanent magnets 2 and 3 since these parmanent magnets have relatively large temperature coefficients of their magnetic flux.
Accordingly, it is eagerly desired to develop a means to improve a permanent-magnet rotary machine with respect to the stability of the output in relation to the temperature variation even when ferrite magnets, rare earth-based magnets and the like are used as the permanent magnets.